The present invention relates to polymeric membranes, and more particularly membranes formed from polyamide-imides, which are useful for separating components of a gas mixture.
The commercial application for gas separation devices based on polymeric materials relies, in part, on maximizing the overall gas flux through the membrane. P. H. Kim, et al., J. Appl. Poly. Sci., 34 1767 (1987), reported that the gas flux for a membrane is relatable to the average space between the polymer chains. In addition, they indicated that the density of the polymer is also related to the overall gas flux. One challenge for commercial applications is to identify polymers which have both very high flux and good thermo-mechanical properties. It has generally been observed that to achieve high overall flux requires having a polymer with low chain-chain interactions. This can be exemplified by polymers such as poly(dimethylsiloxane) or poly(4-methyl-1-pentene). These materials have rather high gas flux values, but due to their low chain-chain interaction, they also have low glass transition temperatures (Tg). As a consequence, these materials require either special processing conditions to build-in chemical and physiochemical crosslinking or else they can only be used at low application temperatures. By contrast, polymers with strong chain-chain interactions have rather high Tg values but typically exhibit low gas flux.
Polyimides, which generally have strong chain-chain interactions and high Tg values, have been reported to have good gas flux values for certain specific structures.
Specifically, U.S. Pat. No. 3,822,202 (1974); Re 30,351 (1980) discloses a process for separating fluids using a semi-permeable membrane made from polyimides, polyesters or polyamides. The repeating units of the main polymer chain of these membranes are distinguished in that such repeating units have at least one rigid divalent subunit, wherein the two main chain single bonds extending from the subunit are not colinear, the subunit is sterically unable to rotate 360.degree. around at least one of these bonds, and has 50% or more of its main chain atoms as members of aromatic rings.
S. Maiti and A. Ray, "Processable Heat-Resistant Polymers. VII. Synthesis and Characterization of Polyamideimide from N-(p-Carboxyphenyl)trimellitimide and p,p'-Di(aminocyclohexyl)methane", J. App. Poly. Sci., vol. 27, 4345-4356 (1982) discloses heat stable polyamideimides; and in "Processable Heat-Resistant Polymers. XIII. Structure-Property Relationship in Polyamideimides", J. App. Poly. Sci., vol. 28, 225-239 (1983) report the relationship between the structure and the properties of heat stable polyamideimides.
Yang, et al., "New Poly(amide-imide)s Synthesis", J. of Poly. Sci., Part A: Polymer Chemistry, vol. 30, 1855-1864 (1992), discloses the synthesis of aromatic poly(amide-imide)s having high inherent viscosities by direct polycondensation reaction of 2,5-bis(4-trimellitimidophenyl)-3,4-diphenylthiophene and aromatic diamines. Yang, et al., U.S. Pat. No. 5,268,487, discloses the preparation of heat resistant poly(amide-ether-imide)s having improved strength and processability.
H. C. W. M. Buys, et al., "Aromatic Copolyimide Membranes for High Temperature Gas Separations: H.sub.2 /CH.sub.4, H.sub.2 /N.sub.2, and O.sub.2 /N.sub.2 ", J. App. Poly. Sci., vol. 41, 1261-1270 (1990) discloses a study done to assess the effect of polymer molecular structure of aromatic copolyimide membranes on the permeability and perm selectivity of gases.
X. Gao and F. Lu, J. appl. Poly. Sci., vol. 54, 1965-1970 (1994) reported a study on structure/permeability correlations for a family of aromatic polyamide-imides. It was reported that the introduction of bulky groups in the polymer chain tended to increase permeability without a corresponding decrease in perm selectivity. The synthesis used was a solid state thermal imidization process using TMAC and a single diamine to form a non-regiospecific poly(amide-imide).
Yokelson, et al., U.S. Pat. No. 5,124,428, discloses heat resistant fiber based on amide-imide resins. The resins were formed by reacting toluene diamine with trimellitic anhydride chloride to form a random polyamide-imide which is solution imidized to greater than 96%.
The work of Fritsch, et al., "Novel Highly Permselective 6F-poly(amide-imide)s as Membrane Heat for Nano-sized Catalysts", J. Memb. Sci., 99, 29-38 (1995), uses the regiospecific PAI materials for gas separation but employs the hexafluoro isopropyl group (either as 6FDA or 6EF44) to ensure solubility characteristics. This increases costs and falls out of the commercially important P/.alpha. ranges.